Device and method for correcting a data error in communication path

ABSTRACT

There are provided a transmission and reception device having a function for correcting a data error in a communication path. In the transmission device, a redundant bit addition unit adds a redundant bit to each data bit which has been divided by one bit by a division unit; and an interleaver performs interleave. The transmission device transmits a signal which has been subjected to FM modulation by an FM modulation unit. In the reception device, a symbol decision unit performs a symbol decision at a Nyquist point for a signal which has been FM-demodulated by an FM demodulation unit; a bit conversion unit performs bit conversion according to the result of symbol decision; and a frame recovery unit deletes the redundant bit added by the redundant bit addition unit of the transmission device, from the bit string de-interleaved by a de-interleaver. Thus, it is possible to surely perform an error correction with a simple configuration even when the communication state is not in a preferable environment.

TECHNICAL FIELD

The present invention relates to a transmission device, a receptiondevice, a data transmission method and a data reception method forfunctioning to correct a data error in a communication path.

BACKGROUND ART

Conventionally, researches on techniques for correcting the data errorin the communication path have been advanced in various quarters. Sometechniques have capabilities close to the Shannon limit.

Particularly, in a mobile communication, since an error characteristicin the communication path changes significantly, a very strong errorcorrection is required.

As to the error correction techniques, retransmission techniques such asARQ (Automatic Repeat Request) and the like, and FEC (Forward ErrorCorrection) techniques are known. The ARQ techniques are that forperforming the error correction by requesting a transmitter toretransmit data which had an error after being received (hence thetechniques are classified into backward). The FEC techniques are thatfor previously devising the data to be transmitted and received so as totransmit reliable data and remove the error at a receiver (hence thetechniques are forward). It should be noted that it is common to use theFEC and the ARQ simultaneously in a data communication, and to use theFEC only when a simultaneous processing on sounds and images isrequired.

However, in a transmission and reception device using the ARQ and thelike, the more retransmissions are increased, the more a transmissionefficiency is decreased. Moreover, in the transmission and receptiondevice using the ARQ and the like, it is difficult to transmit andreceive voice data or image data transmitted through a telephone call orstreaming, due to necessity of the simultaneous processing.

For this reason, with respect to the data communication for e-mails andthe like, and the transmission of the voice data or the image data, atransmission and reception device using the FEC techniques and the likehas been proposed for restoring the received data as far as possiblewithout retransmitting the data by performing the error correction forthe received data, as described in Japanese Patent Laid-Open No.2002-3444413 (pages 6 to 8,FIG. 1). This transmission and receptiondevice performs the error correction using block codes or convolutioncodes.

However, in the transmission and reception device using the conventionalFEC techniques, computation of the error correction is so complex thatcomputation processes are increased. For this reason, in such atransmission and reception device, a significant memory capacity is alsorequired for the computation.

Moreover, the transmission and reception device of a FEC method causes adisadvantage in which on the contrary more errors are occurred when thedata errors increase in the communication path beyond a processingcapacity for the error correction.

Particularly, in the communication such as a sound call, such adisadvantage is not preferable. Since the sound has many factors to becaptured in a human sense, it is more important to be able to recognizewhich words are spoken in the sound even including some noises. In otherwords, if the FEC causes many more errors, data interpolation, repeat ordiscard (missing) and the like are performed. This process is referredto as Bad Frame Masking process. When this Bad Frame Masking processoccurs frequently, the contents itself of the call would be likely to beinaudible.

DISCLOSURE OF THE INVENTION

The present invention has been made in view of these conventionalissues, and an object of the present invention is to provide a technicalmethod capable of surely performing an error correction, and atransmission device, a reception device, a data transmission method anda data reception method employing the technical method.

To achieve the above described inventive object, a transmission deviceaccording to the first viewpoint of the present invention basicallyconsists of a redundant bit addition unit for adding redundant bit datato each bit of provided data to generate coded data, and a modulationunit for sending a modulated wave signal which has been generated basedon the coded data generated by the above described redundant bitaddition unit.

In the transmission device of the present invention, the above describedredundant bit addition unit preferably arranges symbols added with theabove described redundant bit data such that a Euclidean distance of thedata added with the redundant bit data becomes large, or adds theredundant bit data to each bit of the above described provided data suchthat a Gray code is generated.

Moreover, the data provided to the above described redundant bitaddition unit is data in which high and low of significance levelsthereof are predetermined, and the above described redundant bitaddition unit may add the redundant bit for the bit data having the highsignificance level of bit-arranged data.

Furthermore, the above described modulation unit may perform modulationaccording to a multivalued FSK method.

A reception device according to the second viewpoint of the presentinvention operates to receive a signal which has been generated based ondata added with redundant bit data such that coded data is generated,and basically consists of a demodulation unit for demodulating the abovedescribed received signal; a symbol decision unit for performing asymbol decision at each Nyquist interval for the signal which has beendemodulated by the above described demodulation unit; a bit conversionunit for converting a symbol value, which has been provided byperforming the symbol decision by the above described symbol decisionunit, into a bit value; and a data recovery unit for composing a datastring by deleting the added redundant bit from the data of the bitvalue, which has been converted by the above described bit conversionunit, to restore original data.

In the reception device of the present invention, the above describedreceived signal is preferably a signal which has been modulatedaccording to the multivalued FSK method, the above describeddemodulation unit demodulates the received signal by converting thereceived signal into a signal of a voltage corresponding to a frequencyof the above described received signal, and the above described symboldecision unit performs the symbol decision by comparing the voltage ofthe signal, which has been demodulated by the above describeddemodulation unit, with preset threshold values.

Moreover, the bit data which has been generated by the above describedbit conversion unit is data in which bits are arranged such that highand low of significance levels thereof are predetermined and the bitdata having the high significance level is added with the redundant bit.The above described data recovery unit may delete the redundant bitadded to the above described bit data having the high significancelevel.

A data transmission method according to the third viewpoint of thepresent invention basically includes the steps of: adding a redundantbit to each bit of provided data to generate coded data; and sending asignal which has been generated based on the above described generatedcoded data.

A data reception method according to the fourth viewpoint of the presentinvention basically includes the steps of: receiving a signal which hasbeen generated based on data added with redundant bit data such thatcoded data is generated; demodulating the received signal; performing asymbol decision at each Nyquist interval for the signal which has beendemodulated; converting a symbol value provided as a result of thesymbol decision into a bit value; and composing a data string bydeleting the added redundant bit from the data of the above describedbit value which has been converted, to restore original data.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a configuration of a transmission andreception device according to an embodiment of the present invention;

FIG. 2 is an illustration showing a configuration of a data frame of avoice vocoder;

FIG. 3 is an illustration showing significance levels of data frames;

FIG. 4 is an illustration showing the contents of an eye pattern and asymbol decision in the case of using a 4-value Nyquist FSK;

FIG. 5 is an illustration showing operations of the transmission deviceshown in FIG. 1;

FIG. 6 is an illustration showing operations of the reception deviceshown in FIG. 1;

FIG. 7 is an illustration showing an error characteristic in thetransmission and reception device shown in FIG. 1; and

FIG. 8 is an illustration showing a relationship between BER and PESQ inthe transmission and reception device shown in FIG. 1.

BEST MODE FOR CARRYING OUT THE INVENTION

A transmission and reception device according to an embodiment of thepresent invention will be described with reference to the drawings.

FIG. 1 shows a configuration of the transmission and reception deviceaccording to this embodiment.

The transmission and reception device according to this embodimentconsists of a transmission device 11 and a reception device 21.

The transmission device 11 for transmitting a signal, which has beenmodulated according to a provided data, is provided with a division unit12, a redundant bit addition unit 13, an interleaver 14, a base bandsignal generation unit 15, a FM modulation unit 16 and a transmissionantenna 17.

In this embodiment, by way of example, the case will be described wherea voice vocoder is transmitted according to a 4-value root Nyquist FSKmethod.

The voice vocoder is a system for representing a sound signal in adigital format, in which a set of parameters of the sound is analyzedand extracted to recompose the sound from the parameters.

Data of the voice vocoder is framed and processed in which informationis delimited in temporal units, as shown in FIG. 2.

The data of the voice vocoder is framed in units of 20 msec. A dataframe of the voice vocoder consists of voice data and error correctiondata, and the number of bits in one frame is to be 72 bits (3600 bps).The voice data is data describing sound information, and the errorcorrection data is data for correcting an error and detecting the errorin the voice data.

The error correction data consists of 5 bits of CRC data, 5 bits of CRCprotection data and 18 bits of sound protection data.

The number of bits of the voice data is to be 44 bits in one frame, andthe number of bits of the error correction data is to be 28 bits.

Each bit data of the voice data has been sorted in a descending order ofa significance level for the auditory sense of human. The voice data isconfigured with 18 bits of protected voice data and 26 bits ofunprotected voice data.

The protected voice data is data of a high significance level to beprotected even when a communication state is not in a preferableenvironment such that many errors are likely to occur. For example, in acommunication such as a sound call, since the sound has many factors tobe captured in a human sense, it is important to be able to recognizewhich words are spoken in the sound even superimposed with noises.

In the case of the transmission of the sound or image, if the error hasoccurred in a high significant bit, the bit is captured in the humansense as a noise irrelevant to the information. In the voice vocoder,such significant data for configuring the sound is treated as theprotected voice data.

For example, in the case of the sound, there are sound pressure data,pitch frequency data and the like in the vocoder data. It is assumedthat the data of the vocoder data consists of 16 bits of the soundpressure data, 10 bits of first pitch information and 10 bits of secondpitch information, as shown in FIG. 3. The FIG. 3 shows that theleftmost bit is a most significant bit (MSB) and the rightmost bit is aleast significant bit (LSB) in respective data. In the example shown inFIG. 3, it is assumed that these bits of the data have been arrangedaccording to the significance level in which shaded bits are highsignificant bits and the most significant bit has the highestsignificance level. It should be noted that the bits considered to havethe high significance level may be previously determined by verifying orsimulating an algorithm of the vocoder and the like.

As the bit becomes higher, the effect on the information by the error inthe bit becomes more significant. For example, in data of “FFFF”, whenthe error occurs in the most significant bit, the data becomes “7FFF”,resulting in a difference of 32768 in decimal number. However, when theerror occurs in the least significant bit, only a difference of 1occurs.

It is also similar with the image data. For example, yellow is generatedby synthesizing red and green. When the error occurs in the mostsignificant bit, the color is changed.

Thus, it is important how to protect the bit data of the highsignificance level. This embodiment protects such bit data of the highsignificance level with a simple configuration.

Back to FIG. 1, the division unit 12 is provided with the data of thevoice vocoder as shown in FIG. 2, and divides the provided data by onebit. As described above, it should be noted that the bits considered tohave the high significance level may be previously determined byverifying or simulating the algorithm of the vocoder and the like, andthat the bits of the voice vocoder data have been arranged in thedescending order of the significance level.

The redundant bit addition unit 13 adds a bit of “1” to the bit of thehigh significance level among respective bit data which have beendivided by the division unit 12, and generates 2 bit data.

The interleaver 14 interchanges between the bits of the protected voicedata and the bits of the unprotected voice data in units of the 2 bitdata, which has been generated by the redundant bit addition unit 13, todistribute the arrangement of the significant bits or the CRC on theframe, and generates a data string for mitigating block errors due tophasing and the like.

The base band signal generation unit 15 generates a base band signalbased on the data string which has been interchanged by the interleaver14.

The FM modulation unit 16 modulates a carrier wave according to the4-value root Nyquist FSK method, with the base band signal which hasbeen generated by the base band signal generation unit 15. The FMmodulation unit 16 is provided with a root cosine filter, and generatessuch a signal in which an eye pattern is formed as shown in FIG. 4, withthe base band signal which has been generated by the base band signalgeneration unit 15. The transmission antenna 17 sends the signal, whichhas been FM-modulated by the FM modulation unit 16, as a radio wave.

The reception device 21 is provided with a reception antenna 22, a FMdemodulation unit 23, a symbol decision unit 24, a bit conversion unit25, a de-interleaver 26 and a frame recovery unit 27.

The reception antenna 22 receives the radio wave sent from thetransmission device 11, and converts the radio wave into a signal of theFSK method.

The FM demodulation unit 23 performs a FM-demodulation by converting thesignal of the FSK method which has been converted by the receptionantenna 22, into a voltage signal of a voltage based on its frequency,and generates a detection signal.

The symbol decision unit 24 performs a symbol decision at a Nyquistpoint of the detection signal which has been generated by the FMdemodulation unit 23. With the detection signal of the FM demodulationunit 23, the eye pattern is drawn as shown in FIG. 4. According to the4-value FSK method, up to three opening portions are observed in thiseye pattern.

This point is set as the Nyquist point, and three threshold values th+,th0 and th− are preset for performing the symbol decision. The symboldecision unit 24 performs the symbol decision by comparing these threethreshold values th+, th0 and th−, with the voltage of the detectionsignal, at the Nyquist point.

When the voltage of the detection signal at the Nyquist point exceedsthe threshold value th+, the symbol decision unit 24 judges that asymbol value is +3. When the voltage of the detection signal at theNyquist point is equal to or greater than the threshold value th0 andequal to or less than the threshold value th+, the symbol decision unit24 judges that the symbol value is +1. When the voltage of the detectionsignal at the Nyquist point is less than the threshold value th0 andequal to or greater than the threshold value th−, the symbol decisionunit 24 judges that the symbol value is −1. When the voltage of thedetection signal at the Nyquist point is less than the threshold valueth−, the symbol decision unit 24 judges that the symbol value is −3.

The bit conversion unit 25 converts the symbol value, which has beenjudged by the symbol decision unit 24, into bits of a bit value based onthe symbol value. As shown in FIG. 4, if the symbol value, which hasbeen judged by the symbol decision unit 24, is +3, the bit conversionunit 25 converts the symbol value +3 into the bit value “0,1”. If thesymbol value is +1, the bit conversion unit 25 converts the symbol value+1 into the bit value “0,0”. If the symbol value is −1, the bitconversion unit 25 converts the symbol value −1 into the bit value“1,0”. If the symbol value is −3, the bit conversion unit 25 convertsthe symbol value −3 into the bit value “1,1”. It should be noted thatthe arrangement of the bits, which have been bit-converted by the bitconversion unit 25, has become a Gray code.

The de-interleaver 26 reinterchanges the data, which has beenbit-converted by the bit conversion unit 25, in units of 2 bits.

The frame recovery unit 27 deletes the redundant bit from the data whichhas been reinterchanged by the de-interleaver 26, and generates theoriginal data frame.

Next, operations of the transmission and reception device according toan embodiment will be described below.

The division unit 12 of the transmission device 11 divides the protectedvoice data including the CRC 5 bits in the voice vocoder data providedas shown in FIG. 5 at (a), by one bit, and generates the bit data eachhaving one bit as shown in FIG. 5 at (b). Moreover, the division unit 12divides the unprotected voice data by two bits.

The redundant bit addition unit 13, as shown in FIG. 5 at (c), adds thebit of “1” to each bit data which has been divided from the protectedvoice data, including the CRC 5 bits, and generates the 2 bit data.

As shown in FIG. 5 at (c), by adding the redundant bit “1” to each bitdata of the protected voice data by the redundant bit addition unit 13,the bit data of the protected voice data certainly would correspond tothe symbol value +3 or −3. In other words, an interval between thesymbol value +3 and the symbol value −3 becomes large, thereby a gain atthe Nyquist point becomes large.

The interleaver 14 interchanges between a pair of the bit added with theredundant bit and the bit of the protected voice data, and 2 bits of theunprotected voice data, in units of 2 bits of the data which has beengenerated by the redundant bi t addition unit 13, and generates the datastring as shown in FIG. 5 at (d).

The base band signal generation unit 15 generates the base band signalbased on the data string which has been interchanged by the interleaver14.

The FM modulation unit 16 modulates the carrier wave according to the4-value root Nyquist FSK method, with the base band signal which hasbeen generated by the base band signal generation unit 15. Thetransmission antenna 17 sends the signal which has been FM-modulated bythe FM modulation unit 16, as the radio wave.

The reception antenna 22 of the reception device 21 receives the radiowave sent from the transmission device 11, and converts the radio waveinto the signal of the FSK method. The FM demodulation unit 23 convertsthe FSK signal which has been converted by the reception antenna 22,into the voltage signal of the voltage based on its frequency, andgenerates the detection signal.

The symbol decision unit 24 performs the symbol decision by comparingthe voltage at the Nyquist point of the detection signal which has beengenerated by the FM demodulation unit 23, with the preset threethreshold values th+, th0 and th−.

The bit conversion unit 25 converts the symbol, which has been judged bythe symbol decision unit 24, into the bits of the bit value based on thesymbol value.

As shown in FIG. 6 at (e), if the symbol value as a result of thedecision by the symbol decision unit 24 is −3, the bit conversion unit25 converts the symbol value into the bit value “1,1”, as shown in FIG.6 at (f). Similarly, the bit conversion unit 25 performs the bitconversion according to the symbol decision value. It should be notedthat the bit arrangement of the bit-converted data has become thearrangement of the Gray code.

As shown in FIG. 6 at (g), the de-interleaver 26 reinterchanges thedata, which has been bit-converted by the bit conversion unit 25, suchthat the data becomes in the data arrangement of pairs of the bit addedwith the redundant bit and the bit of the protected voice data, and the2 bits of the unprotected voice data.

The frame recovery unit 27 deletes the redundant bit added to theprotected voice data, from the data which has been reinterchanged by thede-interleaver 26, as shown in FIG. 6 at (h), and composes therespective bits to generates the original data frame as shown in FIG. 6at (i).

When focusing attention only on the protected bit data, as a result, thetransmission device 11 has performed a 2-valued modulation instead of a4-valued modulation. Moreover, the reception device 21 only deleteslower bits, and as a result, the process performed by the receptiondevice 21 would be equivalent to performing the demodulation of the twovalues.

Therefore, though each symbol interval is “2” in the case of 4 values,the symbol interval would be “6” which is three times as many as 2, andin theory, BER would be improved by approximately 4.8 dB, according tosuch configuration of this embodiment.

Thus the transmission device 11 adds the redundant bit according to the4-value FSK method, and the reception device 21 deletes the redundantbit which has been added by the transmission device 11. As a result,though it has been equivalent to a 2-value FSK method in terms ofcharacteristics, the modulation method remains to be the 4-value FSKmethod.

BER curves in the case of focusing attention only on the protected bitsare shown in FIG. 7.

In FIG. 7, a characteristic curve L10 represents a characteristic by thetransmission and reception device according to this embodiment. Acharacteristic curve L11 represents the characteristic in the case wheredecoding has been performed by a Viterbi decoder having an encodingratio of 1/2. A characteristic curve L12 represents the characteristicin the case where the error correction is not performed. Moreover, theright edge of a graph represents the case where the communication stateis most preferable, and the graph represents that the communicationstate degrades as the value of Eb/No changes toward the left side.

As shown in this FIG. 7, if the communication state is preferable, asshown with the characteristic curve L11, the effect of the errorcorrection by decoding with the Viterbi decoder is larger, and the BERis low. However, as the communication state degrades, the effect of theerror correction by decoding with the transmission and reception deviceaccording to this embodiment would be larger in the effect of its errorcorrection capability, than the case of decoding with the Viterbidecoder.

Moreover, sound quality characteristics are shown in FIG. 8, in the casewhere the transmission and reception device according to this embodimentis applied to an actual vocoder.

For evaluation of a sound quality, PESQ (Perceptual evaluation of speechquality) is used which is recommended by the ITU-T. It should be notedthat in FIG. 8, L10, L11 and L12 represent the characteristic by thetransmission and reception device according to this embodiment, thecharacteristic by the Viterbi decoder, and the characteristic in thecase where the error correction is not performed, respectively,similarly to FIG. 7.

As shown in FIG. 8, if the BER is low, in other words, if thecommunication state is preferable, the sound quality by decoding withthe Viterbi decoder is more preferable than with the transmission andreception device according to this embodiment. However, this differenceis slight, and it is not a discriminable difference when the sounds areactually heard and compared. If the BER is high, in other words, thecommunication state is not preferable, the sound quality by decodingwith the transmission and reception device according to this embodimentwould be more preferable than with the Viterbi decoder, and the highsound quality may be realized.

As described above, according to this embodiment, the transmissiondevice 11 adds the redundant bit to each data bit of the voice vocoder,and after interleaving, FM-modulates and transmits this signal. Thereception device 21 performs the symbol decision after performing theFM-demodulation, performs a bit-conversion de-interleaving, and thendeletes the redundant bit added by the transmission device.

Therefore, even when the communication state is not in the preferableenvironment, it is possible to more surely perform the error correction.Particularly, the transmission and reception device of this embodimentwould be suitable for transmission of the sound or the image through atelephone call or streaming.

Moreover, the error correction is performed by performing a simpleprocess, in which the transmission device 11 adds the redundant bit tothe data and the reception device 21 deletes the redundant bit of thedemodulated data. Therefore, compared to a FEC method in which manyoperations are performed, and using the Viterbi decoder and the like inwhich a significant memory capacity is required, it is possible to havethe simple configuration, since neither the operations nor the memorycapacity for the error correction is required. In addition, it ispossible to realize to have a lower electrical power consumption, sinceit is not necessary to have a processor operate on high speed.

It should be noted that various forms may be conceived in implementingthe present invention, and they are not limited to the above describedembodiments.

For example, in the above described embodiments, the case has beendescribed where the sound call is performed by using the 4-value rootNyquist FSK. However, the data to be processed is not limited to thevoice data, and the data may be the image data. The FSK is not limitedto 4-value, and it may be multivalued which is equal to or more than 4values. In addition, not only the FSK, but also other modulation methodssuch as PSK and the like may be used.

Moreover, in the above described embodiments, the example such as thetelephone call or the streaming has been described in which the bitsignificance level is defined. However, the present invention may alsobe well applied to the case where it is desired to raise the gain simplyalso in a protocol or an e-mail communication.

Moreover, this embodiment may be executed by means of software. In thiscase, the transmission device 11 and the reception device 21 areprovided with processors for executing the software. Even if thisembodiment is executed by means of software, since it is not necessaryto perform such operations as with the FEC, programs become simple, andit is possible to reduce the memory capacity required for the programs.

In this embodiment, the voice vocoder has been described by way ofexample. However, this embodiment may be applied not only to the voicevocoder, but also to a data communication. In this case, portions ofdata desired to be protected strongly and other data may be applied tothe protected data and the unprotected data in this embodimentrespectively.

Moreover, in the data to be used in the data communication and the like,the number of bits may be changed each time the contents of thecommunication change. In addition, for example, as is the case where“FF” and “FE” are flags for denoting the transmission and the receptionrespectively, even the least significant bit may also have the samesignificance level as an upper bit. In such a case, this embodimentwould become significantly effective, for example, if 3 bits of controlflag is added to the end of the data to make only these 3 bits strong inthe error and the significance level may be defined accordingly.

Moreover, in this embodiment, the redundant bit addition unit 13 hasadded the redundant bit data to each bit of the provided data such thatthe Gray code is generated. However, it is not limited to the abovedescribed embodiments, if the redundant bit addition unit 13 arrangessymbols added with the redundant bit data such that a Euclidean distanceof the data added with the redundant bit data becomes large.

Industrial Applicability

According to the present invention, it is possible to provide atransmission device and a reception device, which can perform an errorcorrection more surely in spite of their simple configurations, evenwhen a communication state in a transmission path is in a defectiveenvironment.

What is claimed is:
 1. A reception device which receives a transmissionsignal in which transmission data of which a level of importance hasbeen previously determined is modulated and transmitted, the receptiondevice comprising: a demodulation unit configured to receive anddemodulate the transmission signal, wherein in the transmission signal,(1) a high-importance data string is divided by one bit to generatehigh-importance bit data each having one bit and added by a fixed-valuebit data of one bit to obtain two bit high-importance bit data and (2) alow-importance data string is divided to obtain low-importance bit dataeach having two bits, and the modulated transmission signal is arrangedto symbols based on the respective two bits, and the fixed-value bitdata is common to, in the arrangement of the symbols, the symbols ofwhich an Euclidean distance is largest, and the symbol including thehigh-importance data string is arranged, by adding the fixed-value bitdata, to one of two symbols, in the arrangement of the symbols, havingthe largest Euclidean distance; a symbol decision unit configured toperform symbol decision at each Nyquist interval for the signaldemodulated by the demodulation unit; a bit conversion unit configuredto convert a symbol value of the symbol obtained on the symbol decisionby the symbol decision unit into a bit value; and a data restorationunit configured to compose a data string by deleting the addedfixed-value bit data from the data of the bit value converted by the bitconversion unit, and restore the high-importance data string.
 2. Areception method which receives a transmission signal in whichtransmission data of which a level of importance has been previouslydetermined is modulated and transmitted, the reception methodcomprising: a demodulation step of receiving and demodulating thetransmission signal, wherein in the transmission signal, (1) ahigh-importance data string is divided by one bit to generatehigh-importance bit data each having one bit and added by a fixed-valuebit data of one bit to obtain two bit high-importance bit data and (2) alow-importance data string is divided to obtain low-importance bit dataeach having two bits, and the modulated transmission signal is arrangedto symbols based on the respective two bits, and the fixed-value bitdata is common to, in the arrangement of the symbols, the symbols ofwhich an Euclidean distance is largest, and the symbol including thehigh-importance data string is arranged, by adding the fixed-value bitdata, to one of two symbols, in the arrangement of symbols, having thelargest Euclidean distance; a symbol decision step of performing symboldecision at each Nyquist interval for the signal demodulated in thedemodulation step; a bit conversion step of converting a symbol value ofthe symbol obtained on the symbol decision in the symbol decision stepinto a bit value; and a data restoration step of composing a data stringby deleting the added fixed-value bit data from the data of the bitvalue converted in the bit conversion step, and restore thehigh-importance data string.